Cation , methanol solutions constants

Chaput, Jeminet and Juillard measured the association constants of several simple polyethylene glycols with Na", K", Cs", and Tl". Phase transfer catalytic processes and most biological processes are more likely to involve the first two cations rather than the latter two, so we will confine the discussion to these. Stability constants for the dimethyl ethers of tetra-, penta-, hexa-, and heptaethylene glycols were determined poten-tiometrically in anhydrous methanol solution and are shown in Table 7.1. In the third column of the table, the ratio of binding constants (Ks/K s) is calculated. Note that Simon and his coworkers have referred to this ratio as the selectivity constant. ... 

Racemization of chiral a-methyl benzyl cation/methanol adducts. The rate of exchange between water and the chiral labeled alcohols as a function of racemization has been extensively used as a criterion for discriminating the Sn2 from the SnI solvolytic mechanisms in solution. The expected ratio of exchange vs. racemization rate is 0.5 for the Sn2 mechanism and 1.0 for a pure SnI process. With chiral 0-enriched 1-phenylethanol in aqueous acids, this ratio is found to be equal to 0.84 0.05. This value has been interpreted in terms of the kinetic pattern of Scheme 22 involving the reversible dissociation of the oxonium ion (5 )-40 (XOH = H2 0) to the chiral intimate ion-dipole pair (5 )-41 k-i > In (5 )-41, the leaving H2 0 molecule does not equilibrate immediately with the solvent (i.e., H2 0), but remains closely associated with the ion. This means that A inv is of the same order of magnitude of In contrast, the rate constant ratio of... 

In our initial studies of the polymerization of butyl acrylate by solid potassium persulfate in acetone solution (2), we attempted to relate the rate of polymerization to the ability of various crown ethers to complex the potassium cation. A reasonable correlation was discovered between log Rp and log K, where K represents the binding constant of the crown ether for in methanol solution (Figure 1). This finding provided some support for the idea that a typical phase transfer process was occurring in these reacti ons. 

In the complexation reaction cryptand must compete with solvent molecules for the cations in solution. Thus solvents such as methanol with low dielectric constant and solvating power offer a preferrable reaction environment but we have achieved quantitative yields in water. The main problem encountered in syntheses of cryptates has been the presence of other cations such as Na and KT competing for the cryptand. Care is taken to minimize the concentration of competing cations of size similar to the cation intended for complexation by using lithium salts for buffering solutions.-... 

The second example of this Section shows instead an opposite behaviour. In this example, the dissociation of the methyl fluoride (CH3F) CF bond in methanol (CH3OH) solution is illustrated. Similar reactions, namely the production of carbocations in polar solvents from alkyl halides, are important in organic synthesis where very often they are favoured by the presence of a catalyst, normally a metal cation. Methanol is a polar solvent the dielectric constant e is 32.6 at 298 K, about half of that of water, but the ratio (e — l)/e is close to unity, showing a strong capability to polarize in a static external electric field. Nevertheless this dissociation is energetically unfavourable. 

An interesting example of the combination of UV and PMR spectroscopies to structure determination in this area is found in the investigation26 of the species present in basic solutions of the 1-methylquinoxalinium cation (10). The UV spectra of dilute aqueous base (pH 10.5) and basic methanol solutions of this cation are very similar lmax = 301, 340 nm (pH 10.5) 304, 344 nm (CH30 /CH30H). In more basic aqueous solutions (pH 14.5), the only absorption maximum is at 347 nm. Two ionization constants (pK = 8.62 and 12.62) were obtained from the pH dependence of the UV spectrum. The PMR spectrum in 3 M NaOD/D20 may be readily assigned to the pseudobase anion 13 however, the PMR spectrum in basic... 

More recently, Stang has devised 46, an optical sensor for Ni, Cd, and Gr, based on a 1,8-Pt-functionalized anthracene, where the organometallic unit acts as a molecular clip, enabling the self-assembly of a rectangular macrocycle.Addition of Ni to 46 in methanol solution resulted in substantial changes in the UVA IS absorption spectrum, due to the 1 1 complexation of the cation. Stability constants were determined for 46 Ni, 46 Gd, and 46 Gr in methanol solution, K = Z.0 x 10, 3.39 x 10, and 7.53 x 10 respectively. 

Confirmation that the side arm participated in complexation when appropriately placed was obtained by comparing the complexation strengths of 8 and 9. Sodium cation is bound in the center of the macrocycle. The side arm of 8 or 9 can reach over the ring, but only 8 has an oxygen donor (methoxy) positioned appropriately to interact. The equilibrium binding constant Ks) for Na by 15-crown-5 at 25°C in methanol solution is 1860. 

In the early years of supramolecular chemistry, a number of studies showed that in comparison to the cyclic crown ethers and bicyclic cryptands, alkali metal cation binding by podands of the oligoethylene glycol type is relatively weak in polar solvents such as water and methanol. Thus [18]crown-6 binds K+ some four orders of magnitude more strongly in methanol solution compared to its open chain podand analog, 2. Representative binding constants for cyclic and acyclic species are shown... 

Rate constants for quenching of 1-7 by methanol and acetic acid in hexane solution from fluorescence quenching and quantum yield data are 10 M l-s-l-. Limiting quantum yields for adduct formation are 0.1. The observation of reactions of protic solvent with 1-7 but not 1-t may reflect the longer lifetime and/or enhanced reactivity of the cyclic molecule. While photo-induced nucleophilic addition is a common reaction of aryl olefins, it is normally observed to occur only under conditions of electron-transfer sensitization (139). Under these conditions, it is the aryl olefin cation radical which undergoes nucleophilic attack. The reaction of 1-7 with protic solvents appears to be the only reported example of nucleophilic trapping of an aryl olefin it,it singlet state (140). 

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